What does a healthy compression test look like?

• A. Each cylinder shows compression within specification and similar across cylinders
• B. One cylinder shows zero compression
• C. Readings vary wildly with no pattern
• D. Only some cylinders are within spec

Answer: (A)

A healthy compression test shows uniform, within-spec pressure in every cylinder. This indicates the pistons, rings, valves, and head gasket are sealing properly and there are no leaks that would rob power. When readings are all within the manufacturer’s spec and the values are close from cylinder to cylinder, the engine is likely in good mechanical shape.

If you see a cylinder with zero compression, that points to a severe seal issue in that cylinder—such as a burned valve, a blown head gasket, or a badly damaged piston or ring. Readings that vary wildly without a clear pattern suggest inconsistent sealing somewhere, which usually means a problem in one or more cylinders. If only some cylinders are within spec, that shows partial issues and warrants further diagnosis to find the exact source of the leak or loss of compression.